1. Field of the Invention
The present invention relates to the field of natural gas powered vehicles, and in particular to the methods for filling the cylinders for such vehicles.
2. The Prior Art
U.S. Pat. No. 4,966,206 to Baumann et al. discloses a device for filling a gaseous fuel container. The Baumann et al. device is configured to adjust the filling procedure to the local ambient temperature, in order to control the pressure of the gas with which the container is filled, toward obtaining a filling of the container, at ambient temperature, which corresponds to the rated filling pressure of the container at a standard temperature. A casing is provided, containing a control device. The ambient temperature, the pressure of the gas to be supplied at the outlet of a compressor, and the pressure differential between the pressure in an inlet line to the casing and the pressure within the casing itself, are all sensed and the control device adjusts the supply pressure of the gas accordingly.
U.S. Pat. Nos. 5,238,030 and 5,259,424 to Miller et al. are directed to filling devices and methods, in which a known quantity of gas is metered into a container. By measuring the stagnation pressure and temperature of the gas, the ambient temperature, and the supply pressure of the gas, the available volume in the container, and thus the mass which can be filled, to the rated pressure of the container, can be determined. The device then meters the calculated amount into the container. Alternatively, the device meters a substantial fraction of the calculated amount, in order to accommodate possible error, and then adds gas in increments, until a cutoff pressure, which is calculated, based upon the ambient temperature, is reached. The pressure within the container is sensed intermittently, during the filling procedure.
Mutter, U.S. Pat. No. 5,029,622 discloses a "slow-fill" system which is configured to accommodate ambient temperature changes which take place during the filling process.
The attainment of accuracy during the filling process for cylinders for natural gas vehicles is important, for the future development of such vehicles, both from a potential commercial standpoint, in order that natural gas "fueling stations" can be operated ill an efficient and commercially viable manner, and from an operator standpoint, so that maximum operating ranges and maximum vehicle safety can be obtained.
Filling accuracy is particularly problematic during fast filling operations (five minutes or less), since the actual filling procedure can have an effect upon the accuracy of the procedure. For example, the ambient temperature must be considered, although once determined, it can be assumed to be constant, due to the speed of the filling process.
As a container is filled, its internal pressure rises during the filling process, and in some filling procedures is monitored to help determine when proper filling has occurred. However, during a fast filling procedure, the temperature of the natural gas within the container rises, which, in turn further increases the pressure within the container. The amount of temperature rise which occurs in the tank has been found to be a function of the amount and pressure of any residual natural gas which is in the container, at the time of filling. Therefore, it is desirable to provide a method and apparatus for the fast filling of cylinders, for example for natural gas, which achieves an improved filling of such cylinders, taking into account environmental conditions, the characteristics of the supply gas, and the characteristics of the cylinder to be filled, including any residual gas remaining in the cylinder.